This invention pertains to the field of semi permeable membranes for separating at least one component of a fluid mixture. More specifically, the present invention relates to composite membranes having a separation layer comprised of a mixture of poly(methyl methacrylate) or a copolymer thereof and at least one cellulosic derivative resulting in the composite membrane having enhanced separation and permeating characteristics. 2. Discussion of Related Art
Permeable membranes capable of separatinq at least one elected component from a fluid mixture, either gas or liquid, are considered in the art as a convenient, potentially highly advantageous means for achieving desirable fluid separation and/or concentration.
To achieve a selective separation, the membrane must exhibit less resistance to the transport of one or more components than that of at least one other component of the mixture. Thus, selective separation can provide preferential depletion or concentration of one or more desired components in the mixture with respect to at least one other component and therefore provide a product having a different proportion of the one or more desired components to the at least one other component than that proportion in the mixture.
However, in order for selective separation of one or more desired components by the use of separation membranes to be commercially attractive, the membranes must not only be capable of withstanding the conditions to which they may be subjected during the separation operation, but also must provide an adequately selective separation of the one or more desired components and a sufficiently high flux, i.e., permeation rate of the permeate per unit surface area, so that the use of the separation procedure is carried out on an economically attractive basis. Separation membranes which exhibit adequately high selective separation, but undesirably low fluxes, may require such large separatinq membrane surface area that the use of these membranes is not economically feasible. Similarly, separation membranes which exhibit a high flux, but low selective separation, may also be commercially unattractive.
Accordingly, work continues to develop separation membranes which can provide both an adequately selective separation of the one or more desired components and a sufficiently high flux such that the use of these separation membranes on a commercial basis is economically feasible.
Membranes are generally utilized for the separation of fluid mixtures in which the feed and the resulting raffinate and permeate do not undergo a phase change during the permeation process.
Membranes are also desired, however, in fluid separation processes that involve a phase change of one or more components of the mixture to be separated. The feed and the permeate streams are thus alternately in the liquid and gaseous state in such processes, with gas being present on one side of the membrane. An example of such a process is pervaporation through membranes, which is particularly useful in the separation of liquids from their azeotrope solvent mixtures, and wherein liquid is present on the feed side of the membrane. Another such process is perstruction, wherein liquid is present on the permeate side of the membrane.
Membranes have been fabricated in various shapes, such as (1) flat sheets which may be supported in a typical plate and frame structure similar to a filter press; (2) flat sheets rolled into spirals with spacing materials interleaved with the membrane and the assembly sealed to provide spiroidal channels permitting the passage of the feed on one side of the coiled membrane to the opposite side of the membrane; (3) as tubes lining the inner surface of a reinforced braid, the braid itself at times being a component in a larger tube; (4) in the form of open ended hollow fibers so organized and sealed into header plates as to provide a separation of the flows over the external surfaces of the hollow fibers from any flow within the bores of the hollow fibers ensuinq by virtue of passage of permeant across the membrane.
Various types of permeable membranes have been proposed in the art for carrying out a variety of fluid separation operations. Such membranes can generally be classified as being of the (1) isotropic, (2) asymmetric, or (3) composite type. The so-called isotropic and asymmetric type membranes are comprised essentially of a single permeable membrane material capable of selectively separatinq desired components of a fluid mixture. Isotropic membranes have the same density throughout the thickness thereof. Such membranes generally have the disadvantage of low permeability, i.e., low permeate flux, due to the relatively high membrane thickness necessarily associated therewith. Asymmetric membranes are distinguished by the existence of two distinct morphological regions within the membrane structure. One region comprises a thin, dense semipermeable skin capable of selectively permeating one component of a fluid mixture. The other region comprises a less dense, porous, non-selective support region that serves to preclude the collapse of the thin skin region of the membrane under pressure.
Composite membranes generally comprise a thin layer or coating of a suitable permeable membrane material superimposed on a porous substrate. The separation layer, which determines the separation characteristics of the composite structure, is advantageously very thin so as to provide the desirably high permeability referred to above. The substrate only serves to provide a support for the thin membrane layer positioned thereon and has substantially no separation characteristics with respect to the fluid mixture being separated or concentrated.
The problems associated with the preparation of composite membranes are many. Most significantly, however, is the challenge of finding a material for use as the separation layer for a given application, which material not only displays high permeablity and high selectivity, but additionally possesses the necessary thermomechanical properties which makes it a good film former to consistently form a continuous thin film without defects or pinholes and be flexible enough for us in a composite membrane.
Accordingly, a need still exists for finding new materials suitable for use as separation layers in a composite membrane having high selectivity without sacrifice in permeation rate, while at the same time having the necessary thermomechanical characteristics necessary for making a thin, continuous film.